1. Field of The Invention
This invention relates to an optical fiber modulator wherein an optical fiber has a poled portion serving as an electrooptic element and having a second-order nonlinear optical effect, and also to a method for making the fiber modulator. The optical fiber modulator of the invention may be applicable to not only as a sensor, but also as an optical switching device and an optical fiber modulator for communication systems.
2. Description of the Prior Art
Known electrooptic elements used in optical fiber sensors for measuring a voltage or in optical fiber modulators are made, for example, of optical crystals of LiNbO.sub.3 (hereinafter referred to simply as LN), Bi.sub.12 SiO.sub.20 (hereinafter abbreviated to BSO), Bi.sub.12 GeO.sub.20 (hereinafter abbreviated to BGO), and the like. According to "Optical Fiber Sensors" (published by Ohm Co., Ltd. and edited by Takayosi Ohkoshi (1986), pp. 149 to 153), optical fiber voltage sensors have high insulating properties, and have been developed especially for the measurement of high voltage.
In recent years, in order to reduce the number of optical elements used in optical fiber sensors, studies have been made on optical fiber sensors of the type wherein lenses and mirrors are omitted from the sensor, and instead, an magnetooptic element or an electrooptic element is assembled in the light path of an optical fiber. This type of sensor is described, for example, in Japanese Laid-open Patent Application Nos. 5-297086, 6-74979, and 8-219825. Quite recently, it has been found that when an optical fiber block is poled, a second-order nonlinear optical effect develops. Using the poled block, optical modulation devices have now been made as described, for example, by A. C. Liu et al in Opt. Lett. Vol. 19, pp. 466-468 (1994), by T. Fujiwara et al in IEEE Photonics Lett. Vol. 7, pp. 1177 to 1179 (1995), and in Japanese Laid-open Patent Application No. 9-230293.
However, with optical fiber sensors or modulators using LN, which is representative of second-order nonlinear optical material, it is necessary that an input beam be controlled so as to make an angle of axial deviation at around 0.1 to 0.2 or below as described in Japanese Laid-open Patent Application No. 3-44562 and "Optical Fiber Sensors" (published by Ohm Co., Ltd. and edited by Takayosi Ohkoshi (1986), page 153).
Problems involved in these sensors are described with reference to FIGS. 8a and 8b. FIG. 8a schematically shows a working principle of a typical optical fiber voltage (or electric field) sensor. In FIG. 8a, a randomly polarized input beam transmitted from an optical fiber is passed to a polarizer wherein a linearly polarized beam component alone is transmitted. The beam transmitted through a .lambda./4 plate is converted to a circularly polarized beam because the phase difference of the beam relative to the respective principal dielectric axes occurs by .pi./2. Further, when passed through an electrooptic element, the beam undergoes a phase difference corresponding to a voltage applied to the element, and is changed in various forms including from circularly polarized beam to linearly polarized beam. This is particularly shown in FIG. 8a as the state of polarized output beam. After transmission through an analyzer, the change in the polarized state is observed as a change in beam intensity. In FIG. 8a, P and t of shaded sketches, respectively, indicate a beam intensity and a time, and the sketches show that a beam with given powder is inputted, and a modulated beam is outputted at the positions depicted, respectively.
FIG. 8b is a graph showing the relation between the beam output strength and the phase difference or optical bias of a fiber sensor. The quantity of transmission of output light or beam is determined by the phase difference of the beam based on an electrooptic effect and an optical bias (determined by a .lambda.4 plate). The output beam intensity is expressed by SIN function. When the optical bias is given by .pi./2 or by a multiple of an odd number of .pi./2, a portion of the SIN function, which exhibits good linearity, can be used. On the other hand, when the optical bias is deviated from .pi./2, e.g. when the optical bias is at 3.pi./4, the output waveform is distorted. In addition, when the optical bias is considerably deviated (e.g. when the optical bias is at zero), not only the output waveform is considerably distorted, but also a degree of modulation becomes very low as is particularly shown in FIG. 8b. With LN, when a beam is passed from its crystal axis (z axis), any birefringence phenomenon does not appear. Only a phase difference of .pi./2 caused by the .lambda./4 plate appears, so that modulation signals, which are free of any distortion, can be obtained as designed. However, when an input beam is deviated from the z axis, a great phase difference appears owing to the great spontaneous or natural birefringence of LN, thereby causing the optical bias to be deviated from an original one. As a result, there arise the problems that the waveform distorts, and the degree of modulation suffers a great temperature change due to the great change of spontaneous birefringence depending on the temperature.
In order to solve these problems, it may occur to use crystals which are free of spontaneous birefringence. Known spontaneous birefringence-free, nonlinear optical materials or crystals include, for example, BGO, BSO, Bi.sub.4 Ge.sub.3 O.sub.12 and the like. However, both BGO and BSO, respectively, have the optical rotary power (i.e. the effect of the plane of polarization being rotated in proportion of the crystal length), so that the crystal length cannot be large, with the attendant problem that the degree of modulation of a beam cannot be optionally set and the degree of modulation cannot be sufficiently increased as described, for example, in the above-mentioned "Optical Fiber Sensors", edited by T. Ohkoshi, pp. 152 to 153. On the other hand, Bi.sub.4 Ge.sub.3 O.sub.12 undesirably involves a DC drift at high temperatures, thus presenting the problem that when used as an optical modulator, a stable temperature characteristic is not ensured. This is particularly reported, for example, by O. Kamada (Jp. J. Appl. Phys. Vol. 32 (1993), pp. 4288 to 4291).
In an optical fiber sensor of type wherein an ordinary electrooptic element is set in position in an optical fiber, any lens is not used. Accordingly, it is necessary to suppress an adverse influence caused by the divergence of a beam, disenabling one to take a sufficiently large crystal length. Accordingly, in case where LN, which has a relatively large electrooptic constant, is used as an electrooptic element, there arises the problem that sensitivity is not enough for use as an optical fiber sensor. Alternatively, if liquid crystals are used, problems are involved in that the response speed becomes very low, an abrupt change of voltage cannot be measured accurately, and such liquid crystals may be solidified when used at low temperatures.
Where part of an optical fiber is poled and used as an electrooptic element, there arises a problem as experienced in the case of a sensor wherein an LN crystal is used as an electrooptic element and an incident beam is deviated from an optical axis (z axis). More particularly, if an optical fiber is poled at part thereof for use as an electrooptic element, not only a nonlinear optical effect (electrooptic effect), but also the anisotropy of refractive index (spontaneous birefringence) develops. When such a poled fiber is used in an optical fiber sensor, it is difficult to obtain an intended fiber sensor. This difficulty does not occur in known optical modulators wherein a change in refractive index based on the electrooptic effect of one of principal dielectric axes is utilized, and in fact, has not been recognized at all up to now.
In an optical modulator proposed, for example, in Japanese Laid-open Patent Application No. 9-230293, the electrooptic effect alone is taken into account, and no mention is made of any optical device as to how to deal with spontaneous birefringence. Accordingly, the resultant modulator has poor linearity. In this instance, two holes are made in the clad portion of an optical fiber so as to insert electrodes thereinto. As a result, there is developed spontaneous birefringence which is ascribed to the anisotropy of the sectional structure of the optical fiber and which is much greater than the spontaneous birefringence developed according to the poling treatment. This optical fiber has such a function as a so-called "polarization-preserving fiber", and the polarized state of a beam inputted from portions other than principal dielectric axes (i.e. a line connecting a pair of holes and a direction normal to the line) becomes very unstable. If such an optical fiber is under varying temperature conditions or is applied with an external pressure thereon, the state of polarization of the beam changes considerably. When this optical fiber is used as an electrooptic element and a beam, which has the direction of poling different from the principal dielectric axes, is inputted to the fiber, the degree of modulation greatly changes by changing a temperature, for example, only by several degrees in centigrade. Thus, the electrooptic element has a very poor temperature characteristic and a large distortion rate.